1. Field of the Invention
The present invention relates in general to a feeding device for feeding a paper sheet or other recording medium on which desired patterns of images or characters are printed by an inkjet print head.
2. Discussion of Related Art
As a feeding device for feeding a paper sheet in an inkjet printer, there is known a device including (a) a drive roller which is disposed on a downstream side of a recording portion of an inkjet print head as viewed in a feed direction of the paper sheet, and (b) a driven roller which is opposed to the drive, roller, so that the drive and driven rollers operate with each other to grip the paper sheet and feed the paper sheet in the feed direction. The driven roller is provided by a gear-like or toothed roller which has a plurality of radially-extending projections formed on a circumference of the roller. The paper sheet can be fed in a predetermined feed direction while being gripped by and between the drive roller and the driven roller which is forced toward the drive roller, after an ink has been ejected onto the medium so as to form an image on the medium. In this instance, even if the ink has not yet being dried, the ink does not adhere to the circumference of the driven roller, owing to the radially extending projections of the driven roller.
Such a known feeding device further includes a drive mechanism which is disposed on an upstream side of the recording portion. The drive mechanism serves to feed the paper sheet from a media supply portion to a printing-start position. When the paper sheet reaches the printing-start position, a printing operation is initiated by successive reciprocating motions of the recording portion in a primary scanning direction (perpendicular to the above-described feed direction). The paper sheet having reached the printing-start position is intermittently fed in the feed direction (corresponding to a secondary scanning direction) by a predetermined distance per each feed motion That is, after each of the successive reciprocating motions of the recording portion, the paper sheet is fed toward the drive roller disposed on the downstream side of the recording portion, while a desired pattern of image is being printed on the paper sheet. When a leading end of the paper sheet reaches the drive roller, the paper sheet enters between the drive and driven rollers. The printing operation is continued while the paper sheet is gripped by and between the rollers. After a trailing end of the paper sheet has passed through the drive mechanism disposed on the upstream side of the recording portion, the paper sheet is fed to a media exit portion, by only the drive and driven rollers cooperating to grip the paper sheet.
Generally, the drive roller is made of a rubber or metallic material. In some cases, the drive roller is covered at its surface with a coating layer which is made of a material having a high degree of coefficient of friction. On the other hand, the driven roller is made of a resin or metallic material. Since the driven roller is forced toward the drive roller, the rollers are in contact with each other during absence the paper sheet between the rollers, thereby problematically causing scratch of the drive roller or its coating layer and wear of the driven roller.
For solving such a conventional problem, there is proposed an arrangement, as disclosed in JP-A-H09-86749 (publication of unexamined Japanese Patent Application), which avoids the contact of the drive and driven rollers with each other. FIG. 12 shows the disclosed arrangement in which an annular groove 43 is formed in an outer circumferential surface of a drive roller 40 which is disposed on a downstream side of the recording portion as viewed in a feed direction of the paper sheet P (indicated by two-dot chain line in FIG. 12). A driven roller 41 is rotatably held by a flexible shaft 42 such that a radially outer end 45 of the driven roller 41 is positioned within the annular groove 43 of the drive roller 40, so that radially outer end portions of the respective rollers 40, 41 intersects or overlaps with each other. When the paper sheet reaches the driven roller 41, the driven roller 41 is forced by a leading end of the paper sheet P to be displaced, against an elastic force generated by the flexible shaft 42, in the upward direction, i.e., in a direction away from the drive roller 40, for thereby permitting the paper sheet P to enter between the drive and driven rollers 40, 41 and to be gripped by and between the radially outer ends 44, 45 of the respective rollers 40, 41. In this arrangement in which the radially outer end 46 of the driven roller 41 is positioned within the, annular groove 43 of the drive roller 41, the rollers 40, 41 are not brought into contact with each other even during absence of the paper sheet P between the rollers 40, 41. The arrangement thus frees the rollers 40, 41 from being undesirably scratched or worn.
In the above-described feeding device or arrangement, after the trailing end of the paper sheet is disengaged from the drive mechanism disposed on the upstream side of the recording portion, the paper sheet is fed to the exit portion, by the grip engagement of the paper sheet and the rollers. For reliably and accurately feeding the paper sheet, the driven roller has to be forced by a predetermined amount of pressing force, onto the paper sheet against the drive roller.
In the arrangement disclosed in JP-A-H09-86749, during absence of the paper sheet P between the rollers 40, 41, the radially outer end 45 of the driven roller 41 is positioned within the annular groove 43 of the drive roller 40, without the radially outer end 45 being in contact with a bottom surface of the groove 43. In this instance, since the rollers 40, 41 are not in contact with the each other, the flexible shaft 42 holding the driven roller 41 does not receive any load, or is slightly flexed downwardly due to weight of the driven roller 41. During presence of the paper sheet between the rollers 40, 41, the flexible shaft 42 is flexed upwardly as a result of the upward displacement of the driven roller 41. In this instance, a restoring force or spring load generated by the flexed flexible shaft 42 acts on the driven roller 41, whereby driven roller 41 is forced onto the paper sheet P.
Therefore, in the arrangement of JP-A-H09-86749, for reliably and accurately feeding the paper sheet P, an amount of the intersection or overlap of the radially outer end portions of the respective rollers 40, 41 has to be sufficiently large, so that the flexible shaft 42 is upwardly flexed by an amount required for obtaining the predetermined amount of pressing force during presence of the paper sheet P between the rollers 40, 41. It is noted that the intersection or overlap amount can be represented by a distance L1 (as indicated in FIG. 12) between the radially outer end 45 of the driven roller 41 and the radially outer end 44 of the drive roller 40.
However, in this arrangement, upon entrance of the paper sheet P between the rollers 40, 41, the leading end of the paper sheet P has to force the driven roller 41 to be raised against the elastic force of the flexible shaft 42 by at least the distance L1. This means that an increase in the above-described overlap amount leads to an increase in force resisting the paper sheet P upon its entrance between the rollers 40, 41. The increase in the resistance force is likely to cause deflection of the paper sheet P and undesirable variation in the distance by which the paper sheet P is fed per each of the successive feed motions, thereby resulting in positional error of the printing spot and other deterioration in the printing quality. Particularly, where the printing operation is carried out by an inkjet printer, namely, where the operation is made with a high value of image resolution, the printing quality could be considerably affected by the variation in the feed distance even if an amount of the variation is small